The invention concerns a method as well as an apparatus for converting from a first delta modulation signal with a first bit rate into a second delta modulated signal with a second bit rate.
Methods and apparatus for converting analog signals into delta modulated signals are well known. However, there are no large systems in operation which are based upon delta modulation apart from limited applications as for example in military communications. Additionally several proposals have been made for using delta modulated signals in digital audio. Until now digital audio has always been based on the pulse code modulation (PCM) with its relatively high hardware complexity. In the case of PCM systems, the problem of converting signals from one sampling rate into signals with another sampling rate has been solved and has led to more or less complex solutions. (In delta modulation, one speaks of a bit rate rather than a sampling rate.) The solutions are particularly complex when both sampling rates are not in an integer and constant ratio. Finally, the conversion from signals in delta modulated form into PCM signals and back is known today but restricted to the case of very simple relationships between the two frequencies.
Until now there is no proposal for a circuit which converts delta modulated signals from one arbitrary bit rate into another arbitrary bit rate. Naturally there is always the possibility of implementing the conversion from delta modulated form to delta modulated form with arbitrary bit rates as well as the conversion between delta modulation form and PCM form and vice-versa via the analog domain. Such solutions, however, lead to complex circuitry and are not economical. As described below, state of the art circuits implementing the conversion via the analog domain may require the use of two integrators with identical or almost identical frequency response, which is expensive. Additionally, conversion implemented via the analog domain does not offer the reproducibility of purely digital methods.